What is a crossover ?

“What is a crossover ?” by 3 square audio is a non technical view to show some of the side effects speaker crossover can impart on the music. “We imagine words passing through the crossover, instead of music, and see the words altered when they exit.”

A crossover is an electrical filter network located inside your speaker cabinet, usually close behind the speaker wire connections.
It accepts the signal and then divides it to each drive unit of the speaker. Using capacitors and inductors the music signal is divided into two halves, high frequencies sent to the tweeter, and low frequencies sent to the woofer.
Crossovers are described as having an 'order' 1st, 2nd, 3rd, 4th etc that denotes strength of attenuation.
It all seems quite harmless and unimportant, it is hidden away and we don't see it working. Speaker manufactures rarely mention them and most reviews take a sideways step. So what is the big deal ? What good or harm can a crossover filter do to the music?
When your music arrived at the speaker it was in pretty good shape. Everything arrived on time, in the correct order and each sound started and stopped precisely where it was meant to. It was correct in transients, amplitude, time and phase.
The speaker drive units, tweeter and woofer, now need all the help they can get from the crossover to maintain the original structure of the music because within the transients, amplitude, time and phase are the triggers that help your ear and brain decide how realistic the music appears to be.
I'm sad to say that once the music has passed through the crossover capacitors and inductors it emerges with changes to its phase, time, amplitude and transients. Every original key part of the musical structure has been altered or moved from its original position. 
Speaker manufacturers do not like talking about their crossover designs very much and if they do talk it is only to focus on the one strength it may possess such as time or phase coherence. They would rather talk about the materials used in the drivers, a bespoke component or how the new speaker betters the previous model and aspects that are more obvious for the buyer to see and understand.
Crossovers deserve more attention and understanding because they set the parameters for the drivers and how they should operate as a 'single' unified force. 

We use 1st order crossover filters for the tweeter and woofer drive units in our speakers. We do this because the music reproduced appears very natural, easy to listen to and understand with effortless detail and a realistic timbre coupled to solid imaging of the sound field.

At 3 Square Audio we are very experienced listeners of music playback systems (35 years and still going strong) and we definitely know how we want our speakers to replay music but only slowly are we beginning to understand why 1st order gives us ALL the properties we want. It is a crossover that can help the speaker drive units, more than any other, work as a team.
In brief it outputs a +45 degree vector to the tweeter and a -45 degree vector to the woofer. This is as good as it gets for the speaker drive units, a signal that sums to unity with a combined phase shift of zero.

Below is a visual version of how the music 'reads' when it exits a crossover that is not 1st order
Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.
The above text shows how the timing and order of information has been corrupted but has sufficient structure for the brain to do corrective work and make sense of it.

The text errors get worse still at the crossover point between the woofer and tweeter. A 2nd order crossover will turn half of the letters upside down while 4th order would not invert any letters or change their order but instead duplicate the first and last letter of each word i.e. ffirstt aandd llastt lletterr ooff eeachh wwordd.
Almost all loudspeaker manufacturers use 2nd, 3rd, or 4th order crossovers and very often a combination, such as 1st on the woofer and 3rd on the tweeter. 
A 1st order crossover will not change the order of letters, turn any upside down, or duplicate them. They will exit the crossover in the same order they entered it. The letters would look tilted, half leaning forward 45 degrees and half leaning back 45 degrees but not upside down, duplicated or in the wrong order.
It is this time/order coherence 'locked' to its phase coherence that allows the brain to turn down its corrective work, relax more and perceive the music signal as more natural and lifelike.
With 1st order we have managed to do the least amount of harm to the music and passed to the tweeter and woofer the closest version of the original. 

It is a great start and this now leads us to the next fantastic step of 'How can the drivers mess it all up ?'
Stay tuned !